The invention relates generally to control circuitry for generating a train of time width modulated pulse signals. More specifically, the invention concerns pulse generation usable with the power switching elements of switched mode power supplies.
Switching mode power supplies conventionally control or regulate output voltage amplitude by varying the length of time during which an input voltage is applied to the primary circuit of a power supply transformer. The removal and application of the input voltage is controlled by an electronic switching element such as a power transistor. To regulate the output voltage of the supply, the duty cycle of the power switching transistor is varied while maintaining a constant pulse repetition rate. Typically the modulation of the pulse width is effected by initiating a pulse with a signal from a clock pulse train generating circuit, generating an error signal by comparing the power supply output to a first reference level, superimposing a periodically increasing waveform such as a sawtooth upon the error signal and comparing the superimposed signal with a second reference level. The pulse is then terminated by the comparator output whenever the superimposed signal exceeds the second reference level.
Typically such power supply regulator circuits have conventionally employed analog comparators for determining the time at which the superimposed error signal/ramp composite exceeds a preselected reference. Such analog comparators are relatively slow, consume excessive power and often require additional suppression circuitry to eliminate unwanted transients (sometimes called "double pulsing") at the pulse width modulation output. Double pulsing is caused by ringing of the superimposed signal about the preselected reference level. Ringing is, in turn, caused by feedback noise generated by the power switches in the supply utilizing the pulse width modulation control. While recently introduced analog comparators exhibit improved operation speed characteristics, they do so only at a substantial increase in price.
Therefore, there is a perceived need for a low cost, high speed, low power consuming comparator for use in a pulse width modulation control circuit not requiring additional circuitry for effecting double pulse suppression.